Is my HVAC Guy correct? 2 systems instead of 1 system with Dampers

I'll soon be working on converting a second floor of a residential building that was never finished and is being used as an attic for the last 80+ years. Building Size 36'x36', 9 1/2' 1st floor ceiling height, 8' planned ceiling height in 2nd floor with plenty of insulation and air barrier planned.  This is also a DayCare Facility.

I will be adding an open stairway to the floor plan to provide access to the 2nd floor. (Clients request for capitalizing on the natural daylighting.)

The HVAC guy informed me that: having 2 separate HVAC systems is the route he would use because-it was how he learned and is the norm in St Louis:

  • Existing HVAC system for  1st floor is undersized 1 1/2 Ton compressor should be 3 Ton unit. (located in the Basement)
  • 1 system with dampers for climate control could  lead to future mechanical problems if one big system was used.  (I don't believe he had faith in a damper system to work over long periods of time.)
  • Controlling Costs and Not disrupting existing downstairs.  We would have to build a chase for the hvac ducts from the basement to the 2nd floor. (closet space is available to do this)

The RED warning light that is going off in my head and is contrary to all I've learned is that: "What will keep one system from robbing the climatized air from the other system?"  The open stairway will allow air movement up and down.

I agree with him partially on 2 systems if there was not an open stairway planned.

Since this is in design stages to capture the natural lighting I've mentioned a French door with a wall would allow lighting and separate the floors.  Which makes a dual system more appealing to me.

Thanks in advance for any advice.

Build Green,


The following CAD drawings were made using Sketchup and what I've found is an easy way to convert plans into picture files for showing my clients what the finished product will look like.  Yes there will be rails on the stairs, omitted so that it doesn't clutter up the drawing.

Instead of this railing set up a wall with a French door would allow the natural light into the room.  And also alleviate my fears of a child climbing the rails and falling to the 1st floor.  If this design is chosen I plan to build the railing 48" tall.

Tags: Air, Energy, HVAC, Movement, Split, Systems

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 By time you have the ductwork oversized enough to handle all the airflow even when only one zone is calling, the zone control system, damper motors, and possibly a 2 stage system you could buy 2 basic single stage systems for the same price.

Oversizing ductwork?  Is this something they did for zoning 30 years ago?  Upstairs/downstairs, needs 2 dampers.  

2 zone Carrier Infinity communicating zoning system is pretty simple and foolproof.  

Here's one I did for Parson's Pipe Organ Co, Infinity Greenspeed with 2 zones - office and toning room:

What was above, what is below:



Here's another cool Greenspeed story: 

What were the costs to install this system?  This isn't a church with unlimited funds available.  Its a small business.  What we do with this project has to make financial sense, for both the near and long term.

It does look like a kick ass system though!  Since the installation have you inquired on what the monthly bills have been?


It's just getting to a year, and will be a tough analysis because they now open the door from the offices to the wood shop during the evening so heating or cooling goes into other parts of the building.  But my experience with ASHP & HybridHeat systems has been savings exceed projections with satisfaction results along the lines of FloridaJoy's. 


We are going to install Ecobee's on other heating units to get a better understanding of how to prioritize future opportunities, hopefully I can get some updates on consumption then.  


The Greenspeed replaced oil, and the rest of the building is on propane heat or low seer window unit cooling, so there is huge leverage in those 3.0 COP/20 seer btu's.  


They've been ecstatic about the comfort.  Because this system manages latent so well "It doesn't feel or smell like Air Conditioning" is a common thread, particularly from those who previously said "I hate air conditioning".  


The greenspeed is nice because it can modulate down so low so giant ductwork isn't needed. However it DID cost at least 2x what 2 simple systems would have. The fact the original systems were right next to each other to begin with makes zoning a lot more cost effective.

Bob, you keep saying such silly things! "2x what 2 simple systems would cost". More than 4 systems, sigh!

Even if it were even remotely close to being true, a Yugo doesn't compare to a pickup truck, particularly when you want to haul drywall and 2 x 4's. Look up the meaning of fallacy and logical argument, it's frustrating addressing poorly considered 'points.'. Comparing solutions to NOT solutions is comparing apples to ducks.

Actually this system was about the cost of two of the crappiest systems available. But that would not have provided the amazing results or satisfaction they are experiencing. And comfortable employees are productive employees.

But please feel free to keep encouraging people to buy that crappy equipment, it exacerbates enclosure and duct deficiencies that better equipment can sometimes hide. More work for me, i solve problems. Keep selling products, I'll come after you and solve problems, or point out what better design could have done.

The wholesale cost on the Greenspeed is simply ridiculous. A basic 14-16SEER condenser can be purchased for $800, a 90% furnace for $600. $1400 per system, $2,800 total for decent equipment. ONE Greenspeed system with the furnace is going to run at least $3k in equipment alone, not counting the additional $500+ for the zoning dampers/module. Sadly the EER isn't much better for Greenspeed than a mid line conventional system... Both hit 12-13 EER even though the Greenspeed costs over 2X the price...

BTW what size furnace/HP did you use and what was the sqft of the building?

The wholesale cost on the Greenspeed is simply ridiculous. 

Irrelevant.  Bob, you keep saying such silly things!  

When compared to flex, hard pipe is CRAZY expensive!  So I guess all your installs are crappy equipment and flex duct?  

Media Filter kits are a lot when you compare to cutting a 1" slot in your return, none of your installs have media filters.  

WOW, LOOK AT THE COST OF ECM motors, they sure add a BUNCH to the cost of a furnace, guess you never recommend them...

Hey Bob, share a picture of your Yugo with us?  You drive a Yugo, right?  

Media filters I do believe in, well worth the $$$ for extended equipment life. Not dust/dirt on the blower/a-coil makes for a lifetime of high efficiency operation. 100% metal ductwork is a tough sell, using metal for plenum/trunks and flex for branch runs gives muck of the benefits of metal at a reasonable cost.

ECM motors are more about comfort than energy savings unless the customer runs the fan 24/7. Payback time exceeds life expectancy of equipment. One out of warranty motor replacement wipes out years of energy savings. Sell ECM for COMFORT, with energy savings secondary.

IMHO most comfort issues come from leaky houses and oversized equipment. Downsize the equipment with a tight house and most of the comfort issues are resolved even when using mid grade equipment. Homeowners don't have unlimited pockets and I think money spent tightening the house is better than spending 5 figures on a HVAC system.

Media filters I do believe in, well worth the $$$ for extended equipment life

I'm getting you to move a little!  Fantastic!  Yes, it is not about one thing, it's about all kinds of interconnected things.  If you only look at one thing, the answer is usually no, and that answer is often wrong.  

Now I just need to move you off this "comfort" vs "efficiency", black OR white viewpoint.  We heat houses so they can be occupied.  If you don't provide an enclosure that can be comfortably occupied, why are you heating?  

Sorry to make the statement "for comfort, not for energy savings" seem stupid, but it is stupid, and too commonly stated.  We need to put a stake through it.  

ECM's save a fortune.  Modulating equipment saves a fortune.  Track and you'll see.  But first you'll have to sell a few - or you won't have anything to track.  

And if you don't track, all your claims about what saves are hypothetical.  You know nothing, everything you say is simply a wild ass guess based upon something someone ELSE told you that you decided to believe without a shred of personal evidence.  Dogma.  Religion.  

I don't Hari Krishna with other people money.  Doesn't meet the BPI standard of "do no harm."

How are mod units getting higher real world AFUE than standard condensing furnaces? Both are about 95%.. Mod and 2 stage units are all about comfort.

Condensing furnaces make sense from a payback prospective due to the gas company rebates and tax credits. Without them, payback would exceed equipment life. From an IAQ prospective condensing furnaces in conditioned space are excellent because they use outdoor air.

ECM's only save big when fan the fan is used on continuous mode. PSC vs. ECM on high speed are close on high speed, on low speed ECM has a clear win vs. PSC.

FWIW I have a 2ton 14.5SEER AC for my 1600sqft house. Highest power bill last summer was $90 with a 75 degree setpoint. How much is that fancy Greenspeed really going to save me on power bills? How long would the payback period be?

I recently installed a 44k 90% furnace to replace my old 88k 80% furnace. Comfort is MUCH better, it will be interesting to see how it affects my gas bill.

That's a good question.  The answer is, if the furnace does not make the occupant comfortable, crazy pet trick chasing comfort ensue.  Furnaces designed to load match tend to provide comfort better than furnaces that speed up to stop lights and shut off.  

I bet your gas bill goes down considerably more than 10%.  Your WHOLE gas bill.  I bet it goes down 25%.  

Your house sounds small enough that mini's might have been a good option.  Getting rid of the gas meter is likely to be a thing those with foresight enough to design for will be very happy to have done in a few years when base meter charges hit $50 a month +.  

No, your situation doesn't sound like it would ever justify a Greenspeed under any comfort or efficiency split.  If you drive 50,000 miles a year that Prius (or Tesla) might justify on cost alone, if you drive 10,000 you better want it for other reasons too.  

Our job, or mine anyway, is to tailor solutions to situations, not get lazy and make cookie cutter "solutions" that fit all situations.  If you are not compensated for design, then recommending cookie cutter solutions may be a good business decision.  I certainly don't agree with those who think you should spend huge amounts of time an energy on design if the customer expects it for free. 

I've looked at going all electric, and gas is still slightly cheaper at his time. Meter charges are $28/mo, but fuel is only $4.67 per Dth. We would have to change water heater, dryer, stove, outdoor grill, and stove to go all electric. We get as much as we can out of the $28/mo meter charge.

25% of the TOTAL gas BILL (does this include the amount for the meter fees, or just consumption?) would mean the new furnace is using about 1/3 less gas than the old one. The cool to the touch PVC flue is evidence that it's wasting a lot less gas than the scalding hot 4" metal pipe the old furnace had.

I'm not sure how much gas furnaces loose in cycling when oversized, and have found no definitive answer online. I was able to change my thermostat from 4CPH to 2CPH and still have better comfort than the old system did at 4CPH. Not sure how much lowering the CPH affects furnace efficiency, but it saves wear/tear on the equipment.


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